Fast reactions stop arcs

5 February 2002

There is a continuing discussion within the power industry about arcing and arc fault protection. The key is fast detection. This can be achieved by using long fibre-optic cables as sensors, in combination with fast-response power electronics based on IGBTs (Insulated Gate Bipolar Transistors). Mats Östergård, ABB Oy, Substation Automation, Vaasa, Finland

Arc faults appear in electrical equipment such as transformers, bus ducts, switchgear, motors, generators, as well as cable splices and terminations, in other words, in all industries where electrical equipment is used.

Although relatively rare, an arc in medium voltage switchgear can be disastrous. An arc can destroy costly equipment, causing long and devastatingly expensive downtime. Furthermore, an arc can cause serious injuries to personnel, and even worse, it can be lethal.

An arc arises when current breaks through insulation and travels between phases, or between phase and earth. Partial discharge in deteriorated insulation can also escalate to high energy arcing when a door is opened or panel cover removed. Sudden ventilation can cause trapped conducting ionised air to pass between buses. In an instant, the momentary peak power may reach levels as high as 40 MW and the temperature may reach 20 000°C, causing an explosively propagating pressure wave. Fire breaks out and the immediate burning effect of the arc melts and burns nearby objects such as cubicle walls, busbars and insulators, which may produce dangerous chemicals. The damage may be extensive. People can get injured or even killed.

Other sources of arcing can be deteriorated solid or liquid insulation, mal-operation of a device, overvoltage, corrosion, pollution, moisture, ferro-resonance (instrument transformers) and even ageing under electrical stress. Again, sufficient maintenance could prevent most of these.

Fast detection

Time is a critical when it comes to detecting and minimising the effects of an arc. An arc fault lasting 500 ms causes severe damage to the installation. If the arc lasts less than 100 ms the damage is often smaller, but if the arc is eliminated in less than 35 ms the damage is almost unnoticed.

Arc detection in combination with protection relays, provides insurance against devastation. The key to total protection is to detect and extinguish the arc as quickly as possible.

Particularly effective in this regard is a technique using a long-fibre sensor (patented by ABB), which detects an arc anywhere within the monitored area.

The long-fibre sensor concept involves running a rugged unshielded fibre-optic cable inside the switchgear enclosure. The fibre senses the intense light from an evolving arc, no matter where it strikes, and thanks to the fast trip time of the relay (<= 2.5 ms) the damage can be minimised. In other words, it achieves full coverage, leaving no area unmonitored.

Loop or radial arrangements of the fibres can be used, whichever is best suited for the switchgear type.

The ABB technology, called REA, is a fast system where the IGBT (Insulated Gate Bipolar Transistor) solid state circuits react instantly isolating the arc long before it can cause serious damage. The fibre detects light along its entire length and provides full protection everywhere, no matter how much of a maze the switchgear interior is.

Light capturing mechanisms

The very same mechanisms that cause light propagation losses in glass fibres (silica core fibres with hard cladding) are utilised in the REA fibre sensor. But in the REA application scattering mechanisms such as bends, slight changes in the core's refractive index (Rayleigh scattering), core not being a perfect cylinder, local density variations and inhomogeneities in the core/cladding layer boundary surfaces (microbends) are used in reverse to catch light entering the fibre from its side into the core, where it starts propagating via total reflections.

Microbends are the most important scattering mechanism, causing light to scatter into random directions, so the angle of incidence does not matter.

Self supervison

The REA system includes a capability for continuous self-supervision. In addition, the system incorporates an integrated detector that prevents unjustified tripping caused by sunrays or a camera flash.

Either a rapid increase of light intensity, or light in combination with subsequent overcurrent is used to trip a circuit breaker.

The system also includes easily implemented selective zone protection. Should it be necessary to extend the scope, extension units can be used to isolate single feeders or bus couplers, maintaining power supply even during an arcing incident. In addition, if one feeder is isolated during maintenance, the arc detection remains intact for the rest of the system.

Protecting the investment

At a fraction of the cost of the switchgear itself, arc detection safeguards the investment, and eliminates potential income losses due to extended outage and production downtime.

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